In many data communication systems data are transmitted by modulating digital data onto transmission signals. Such data transmission systems include single-carrier data communication systems such as QAM (Quadrature amplitude modulation) or PSK (Phase shift keying) and multi-carrier system using a plurality of subcarriers such as OFDM (Orthogonal frequency division multiplexing) or DMT (Discrete multitone modulation) to transmit data on multiple frequency bands.